FIG. 1 shows part of a known pump assembly for use in a common rail diesel engine. The pump assembly 10 includes a pump housing 12 provided with a blind bore 14 within which a pumping plunger (not shown) reciprocates, in use, under the influence of a drive arrangement (also not shown). The plunger and its bore extend co-axially through the pump housing 12 with the blind end of the bore defining a pump chamber 18 for fuel. Fuel at relatively low pressure is delivered to the pump chamber 18 through an inlet passage (not shown) under the control of an inlet non-return valve 20. Fuel is pressurised within the pump chamber 18 as the plunger reciprocates and, once it reaches a predetermined level, is delivered through an outlet valve in the pump housing (not shown) to an outlet passage which extends transversely to the bore 14. The outlet passage delivers pressurised fuel to a downstream common rail.
The pump housing 12 is provided with a cover 22 which is fixed to the pump housing by means of bolts (not shown). The cover 22 is of generally top-hat construction, having an annular skirt 22a which engages with an upper surface of the pump housing 12 and through which the bolts are located. By shaping the cover in this way and by locating the bolts through the annular skirt 22a around the periphery of the pump assembly, the overall profile of the pump assembly is more compact than in alternative arrangements in which a cover is mounted axially above the pump housing and bore. This arrangement thus has advantages in terms of space efficiency, which is of significant benefit in the crowded engine space. The remaining underside of the cover 22 and the upper surface of the pump housing 12 together define a volume 28 for receiving low pressure fuel which acts as a reservoir from which fuel is drawn through the inlet passage to the pump chamber 18 when the inlet valve 20 is open. The cover 22 also provides a protective feature for the pump assembly components.
Due to the high pressures that are generated within the pump chamber 18 during the pumping cycle, one problem that may occur within the pump assembly of the aforementioned type is high pressure fatigue of parts. As the plunger reciprocates within its bore 14 and fuel is pressurised to a high level within the pump chamber 18, a pulsating tensile stress occurs within the pump housing 12 that can cause cracks to grow. The pulsating tensile stress has two main effects within the pump housing 12: hoop stress acts around the perimeter of the plunger bore 14, particularly in the vicinity of the pump chamber 18, and axial stress acts along the length of the plunger bore 14. Therefore, it would be a benefit to have a pump assembly of the above-described type in which pulsating tensile stress and high pressure fatigue are reduced or eliminated.
GB2107801 describes another type of fuel injection pump, in which a pump housing is provided with a bore through which a piston reciprocates. A pump chamber is defined at one end of the piston bore, and two axial bores (one fuel inlet—for low pressure fuel, and one fuel outlet—for high pressure fuel) extend from the side of the pump chamber opposite the piston to the upper surface of the pump housing. A valve head (or block) is seated directly against the upper surface of the pump housing and secured in place with threaded screws so that a sealing engagement is formed between the pump housing and the valve head, which guards against fuel leakage between the two components. In this arrangement, the mounting of the valve head directly above the upper surface of the pump housing substantially counteracts any tensile stress caused by fuel pressure in the pump chamber. However, the inherent benefits in countering hoop and axial stress in this arrangement are at the cost of assembly size, in particular length (or axial height), which places additional packing constraints on the already crowded engine space. Also it is notable that in this arrangement the valve head contains an outlet valve member that is required to control the flow of high pressure fuel (possibly in excess of 2000 bar pressure). This configuration has the further disadvantage that the high pressure fuel generated in the pump chamber can potentially find a leakage path at the interface of the valve head (or block) and the pump housing. Fuel leakage is detrimental to pump performance and, therefore, it would be desirable to mitigate any risk of or actual fuel leak.
Thus, having regard to the prior art, it would be an advantage to have a fuel pump assembly of reduced size and of a convenient shape for engine packing requirements. It would further be beneficial to have a fuel pump assembly in which fuel leakage is reduced or minimised. It would also be desirable to have a fuel pump assembly in which axial stress and/or hoop stress is reduced or eliminated. Thus the present invention aims to reduce and/or solve one or more of the problems in the prior art.